In U.S. Pat. No. 4,555,940, an apparatus for measuring and for monitoring the volume and rate of liquid flow through a flow path is disclosed. The apparatus disclosed in U.S. Pat. No. 4,555,940 includes a means for modifying the temperature of a part of the liquid flow path. After a fixed amount of heat is delivered, and the temperatures of the liquid flow path and the liquid therein have been sufficiently modified, a temperature change detector means is activated. Said detector means monitors changes in temperature as a function of liquid flow rates through a temperature modified portion of the liquid flow path. The apparatus according to U.S. Pat. No. 4,555,940 can detect bubbles by employing the fact that the temperature of the temperature modified portion of the liquid flow path will increase when bubbles pass through compared to temperature when only liquid is flowing through said the temperature modified portion, provided the amount of heat transferred to the temperature modified portion of the liquid flow path remains substantially unchanged.
The techniques disclosed in U.S. Pat. No. 4,555,940 are not capable of detecting the presence of bubbles in gas phase when alterations of liquid flow velocity are conceivable. Namely, the techniques disclosed in U.S. Pat. No. 4,555,940 cannot discriminate between the various causes of temperature changes of either the liquid flow path or the liquid therein. That is, provided the amount of heat transferred to the temperature modified portion of the liquid flow path remains substantially unchanged, a temperature change is attributable to a change in liquid flow rate as well as a presence of bubbles. As a result, no appropriate action can be deployed based on the registered temperature change.